To improve safety of battery, heat-resisting non-woven fabric and wet porous membrane have been considered recently. Non-woven fabric has a high porosity and a long diameter since it is fiber assembly. However, it is relatively difficult to reduce the porosity of the wet porous membrane with maintaining connection between adjacent spaces, because the porous member is obtained by elution of fine particles mixed within a deposition layer or a membrane. In addition, low porosity would cause cost increase since a lot of expensive resin needs to be used. Furthermore, although high porosity is advantageous for charging and discharging at high rate, if once lithium deposits for example in quick charging at a low-temperature, an problem would happen that metal lithium become easier to penetrate inside. To solve this, laminating a porous layer such as polypropylene (PP) is possible, but there are problems such as increase of separator thickness and increase man-hours and cost for manufacturing a fine porous membrane.
With respect to a separator, patent document 1 discloses a separator with a low thermal contraction in overheating as following. The separator comprises a porous substrate made of material where its melting point is 180° C. or more, and a resin assembly formed at least one side or inside of the porous substrate, wherein the separator further included filler particles which has a melting point of 180° C. or more or substantially has no melting point.